two possibilities - thermostat stuck open or temperature sending unit.

My 1991 560 SEL had almost the same thing as you descriped, temp gauge showing about 68 C even after a long drive. Almost every one said thermostat but it actually was the sending unit. If you have a DMM, do a very quick OHM measure on that unit. Do a search on my id and you will find a chart of temp and ohm. If teh ohm is right, then it is thermostat for sure. These car's thermostats' failing mode is open rather than close.